Method of handing over a call set up with a mobile station from one cell to another within a cellular radio network

ABSTRACT

The safe method of handing over a call set up with a mobile station (M) comprise the following cyclic steps: 
     a following base station (B(n)) receives (ET3) UP data (DM) of the call in a following channel (CH2), which UP data is transmitted by the mobile station (M) in a current channel (CH1) to a current base station; 
     the DOWN data of the call is transmitted (ET5) for a given duration (T) simultaneously both over said current channel (CH1) and over said following channel (CH2), from the current base station (B(n-1)) and from the following base station (B(n)) respectively as soon (ET4) as the propagation time difference (TA) between the respective instants at which said UP data is received by said current channel (CH1) and by said following channel (CH2) is less than a first predetermined threshold (S); and 
     on expiry of said given duration (T), releasing (ET6, LIB) said call in said current channel, and validating said call in the following channel (CH2) as a new current channel.

The present invention relates in general to a method of transferring acall set up with a mobile station in a digital cellular radio networkfrom one cell to another. Such a transfer is generally referred to as a"handover". More particularly, the invention relates to a safe handovermethod for use in a transport network for handing over a call set upwith a mobile station from a current channel associated with a currentbase station to a following channel associated with a following basestation. The method is implemented along a mobile station path which isassociated in deterministic manner with successive base stations in aradio network for communicating with mobile stations. In addition, thenetwork is "synchronized", which means that the base stations in thenetwork are synchronized with one another ignoring a substantially zeropropagation time difference. The unit time intervals for conveying datain the transmit and receive channels for two arbitrary base stations ofthe radio network coincide.

In a cellular digital network of the GSM (global system for mobilecommunication) network, the handover of a call from a current channel toa following channel appears as a critical stage in call management. Sucha handover can give rise to two types of momentary break in the call,referred to as a "synchronization" break and as a "power" break. Amomentary synchronization break, which typically lasts for about 20 msin GMS, stems from synchronization of the mobile station. The mobilestation is synchronized in time and in frequency on the current channelprior to handover and it needs to resynchronize onto the followingchannel after handover. During the time required by the mobile stationto achieve new synchronization, the call is momentarily interrupted. Apower break is the result of the fact that when the mobile station ischanging cells, it is located at maximum acceptable distances both fromthe current base station and from the following base station. In anotional zone at the boundary between two cells, the power of datareceived by the mobile station in said zone from each of the basestations concerned, i.e. the current station and the following station,is therefore at a minimum level. Radio propagation phenomena can thuscause the mobile station to cease receiving call data momentarily.

The prior art provides for remedying the causes of call synchronizationbreaks by providing a synchronized network in which the current andfollowing channels concerned with a handover of a particular call areidentical. In GSM, this identity of current and following channels givesrise to the same time interval and the same frequency. However, itprovides no way of mitigating "power" breaks in a call that is set up.

A main object of the invention is to provide a safe method ofchannel-to-channel handover for a call set up with a mobile station in aradio network, that provides an effective remedy against power breaks ina call.

To this end, the invention provides a method of safely handing over acall set up between a mobile station using a current channel associatedwith a current base station to a following channel associated with afollowing base station along a path in a transport network for saidmobile station, which path is associated in deterministic manner withsuccessive base stations in a radio network for communication withmobile stations, the method being characterized in that said current andfollowing channels are identical, and in that it comprises the followingcyclical steps:

reception by at least one previously activated base station of UP dataof said established call in said following channel as transmitted insaid current channel by said mobile station;

measuring a propagation time difference between the respective instantsat which said UP data is received by the current base station in saidcurrent channel and by the following base station in said followingchannel;

simultaneously transmitting, during a given duration, DOWN data for saidcall over said current and following channels respectively from saidcurrent base station and said following base station, as soon as saidpropagation time difference is less than a first predeterminedthreshold; and

on expiry of said given duration, releasing said call in said currentchannel, and validating said call in the following channel as a newcurrent channel.

In a first variant, the given duration is a predetermined fixedduration. In a second variant, the given duration expires as soon as thepropagation time difference exceeds a predetermined threshold.

Advantageously, said following base station is determined as a functionof said current base station, as a function of source and target basestations between which unsafe cell to cell handover is performedstarting from said source base station through which the call is set up,and as a function of a base station sequence stored in theinfrastructure of said radio network.

In practice, two opposite direction paths are defined for a length ofthe transport network in which the invention is to be implemented. Undersuch circumstances, the unsafe handover of the call to the target basestation is performed on a channel selected from a respective one of twosets of channels as a function of said travel direction.

The successive base stations of the radio network which are associatedin deterministic manner with the path of the mobile station comprise allof the base stations between the target base station towards which theunsafe channel-to-channel handover is performed and a last base stationassociated with a node or an end of the transport network.

Other characteristics and advantages of the present invention appearmore clearly on reading the following description given with referenceto the corresponding accompanying drawings, in which:

FIG. 1 shows the cells of successive base stations covering a portion ofa transport network along which mobile stations travel;

FIG. 2 shows the steps of the method of the invention; and

FIG. 3 is a graph showing propagation time difference for data receivedby two adjacent base stations and coming from the same mobile station,as a function of the position of the mobile station.

It is important to observe that application of the safechannel-to-channel handover method of the invention for a call alreadyset up with a mobile station is restricted to a mobile station thatfollows paths in a transport network of the "linear" type, typically arail network or a motorway network. In the context of the invention, thedefinition that needs to be given to the term "path" is defined by theinterpretation that results from the text below. Each path in a lineartype transport network is associated in deterministic manner withsuccessive base stations of a radio network for communicating withmobile stations. This notion is explained with reference to FIG. 1 whichshows a portion of a rail transport network, typically a TGV network(train a grande vitesse=high speed train). The portion shown of thenetwork comprises a node C and three ends A, B, and D which areassociated, in practice, with rail stations. Two opposite directiontracks CD and DC interconnect the stations C and D, two oppositedirection tracks CB and BC interconnect the stations C and B, and twoopposite direction tracks CA and AC interconnect the stations C and A.The link between stations C and D is covered by M=6 base stationcoverage areas C100, C30, C31, C32, C33, and C34; the link betweenstations C and B is covered by M=5 base station coverage areas C100,C10, Cll, C12, and C13; and the link between stations C and A is coveredby M=5 base station coverage areas C100, C20, C21, C22, and C23. All ofthe cells are of respective dimensions that are substantially identical.In practice, the differences between the dimensions of two respectivecells is limited by tolerance determined by the technique ofequalization filtering used in the mobile station. Equalizationtolerance is defined as a maximum acceptable propagation time differencebetween two identical signals received by the terminal. For apropagation time difference that is less than the maximum acceptablepropagation time difference, the purpose of equalization is to absorbsaid propagation time difference and thus, in the prior art, to remedythe problem that results from multipath propagation. For GSM terminalsit is typically 20 microseconds. The definition of a path to which themethod of the invention can be applied is specified below. For a giventrain provided with a mobile station starting from the rail station atnode C and heading towards the rail station at end D, a call isestablished on departure of the train with the mobile station in achannel conveyed by a source base station associated with the sourcecoverage area C100. The destination of the train is not known a prioriby the mobile network and, as a result of the train moving, the firstcall handover from the cell C100 to the target cell C30 is performed inconventional manner, i.e. in a manner that is not "safe". Typically, inthe context of GSM, such an unsafe handover is the result of the mobilestation transmitting respective power levels for the beacon frequencysignals transmitted by the adjacent base stations. The infrastructure ofthe radio network or "mobile" network typically controlling the basestation or switching the mobile service and receiving such measuredpower levels takes a decision to transfer the call to the target basestation C30 from which the mobile station receives the beacon frequencysignal with maximum power. In accordance with the invention, the mobilenetwork infrastructure stores the chaining or sequence of base stationsfor each of the links between A and C, B and C, and D and C. As a resultof this unsafe handover of the call from cell C100 to cell C30, themobile network infrastructure can deduce which cells C31 to C34 will bepassed through in succession by the mobile station, since it is knownfirstly from the unsafe transfer which direction the mobile station istaking and secondly, from the stored sequence, which stations are to befound in succession in that direction. Thus, for a train leaving C andheading towards D, the path of the mobile station can be associated indeterministic manner with successive base stations of the radio networkstarting from the mobile station being located in cell C30, followingthe unsafe transfer and terminating in the rail terminus station C34. Inanother example, for a train leaving D and heading towards A, the pathof the mobile station can be associated deterministically withsuccessive base stations of the radio network starting from the mobilestation being located in cell C33 following unsafe transfer from cellC34 to said cell C33, and terminating with the mobile station beinglocated in the cell associated with rail station A. In practice, thesuccessive base stations of the radio network which are associated indeterministic manner with said path of the mobile station comprise allof the base stations between the target base station towards which afirst channel-to-channel handover takes place in unsafe or conventionalmanner, starting from a source base station through which the call isset up, and ending at a last base station associated with a node or anend of the transport network.

Apart from the case of an unsafe cell-to-cell handover being performedstarting from a first or source base station through which the call isinitially set up, and transferring to an unforeseen following basestation, the subsequent following base station, or plurality offollowing base stations at a node rail station, can be determined in theinfrastructure as a function of the current base station, of the traveldirection of the mobile station as defined by the source and target basestations between which said unsafe cell to cell handover took place, andof the sequence of base stations as stored in the infrastructure of theradio network. This ability to determine the following base station(s)as a function of the current base station makes it possible to implementthe method of the invention, as described below with reference to FIG.2.

Initially, in a first step ET1, UP data transmitted by the mobilestation M to the infrastructure 1, and DOWN data transmitted from theinfrastructure 1 to the mobile station M for the call COM conveyed in acurrent channel CH1 passes via a current base station B(n-1), of rank(n-1). In step ET2, as soon as said call COM is conveyed via basestation B(n-1), the infrastructure 1 activates ACT on receiving thefollowing base station B(n) of rank n on the following channel CH2 whichis identical to the current channel CH1. In a digital network, identitybetween channels CH1 and CH2 means that they have identical frequenciesor identical frequency patterns in the frequency hopping technique, andidentical time intervals. The following base station B(n) is determinedby the infrastructure as described above. In response, in step ET3, thecall is still conveyed over the channel CH1 to the base station B(n-l),and additionally UP data DM of the call as transmitted by the mobilestation M is received by the following base station B(n) that isactivated to receive. Because of the difference between the twodistances between the mobile station M and the current base stationB(n-1) and the following base station B(n), the UP data of the call COMas received by the infrastructure 1 via the current base station B(n-1)and the same UP data DM for the call COM as received by theinfrastructure 1 via the following base station B(n) is subject to apropagation time difference TA. This propagation time difference TA isequal to the difference in the propagation times between the mobilestation and the two base stations B(n-1) and B(n). In step ET4, theinfrastructure 1 determines whether this measured propagation timedifference TA is less than a predetermined threshold S. As soon as saidpropagation time difference TA is less than the predetermined threshold,provision is made for the DOWN data of the call to be transmittedsimultaneously for a given duration T towards the mobile station M overboth said current channel CH1 and said following channel CH2 by saidcurrent and following base stations respectively (step ET5), it beingunderstood that DOWN data is already being transmitted over the channelCH1. In addition, power measurements and quality measurements on thesignals received by the current and following base stations B(n-1) andB(n) respectively over the current and following channels CH1 and CH2may also be performed to validate the step of undertaking simultaneoustransmission. In this way, a transmission diversity technique isimplemented by the base stations B(n-1) and B(n), thereby guaranteeingbetter quality of call reception by the mobile station M. In addition,such transmission diversity is implemented only for the given duration Tso as to avoid interference between data transmitted over the currentchannel and the following channel. In practice, this given duration T issuch that the respective propagation time differences between the mobilestation M and the base station B(n-1) and between the mobile station Mand the base station B(n) during said given duration T is not excessive.Two variants are proposed for selecting the given duration T. In a firstvariant, the given duration T is a predetermined fixed duration definedin the infrastructure 1. In a second variant, this given duration Texpires as soon as the propagation time difference TA crosses apredetermined threshold that may be identical to the predeterminedthreshold S that triggers simultaneously transmission. FIG. 3 shows thissecond variant. It is in the form of a graph showing the value of thepropagation time difference TA as a function of the position x of themobile station M relative to the current base station B(n-1) and thefollowing base station B(n). Simultaneous transmission is triggered assoon as the propagation time difference TA crosses the threshold S, withthe mobile station being in a position x1, and it is interrupted as soonas said propagation time difference moves back above said threshold S,when the mobile station M reaches position x2. After the given durationT has expired, the infrastructure releases, LIB, the current channel CH1associated with the base station B(n-1), and the call COM is validatedin the following channel CH2 associated with base station B(n). The callCOM in the following channel CH2 must be uniquely validated by theinfrastructure 1 since this channel CH2 conveys the UP data DM from themobile station to the infrastructure 1 (step ET3) and the DOWN datatransmitted from the infrastructure to the mobile station M (step ET5).Given that the method of the invention is iterative by nature, as shownby step ET7 in which the index nis incremented by one, the base stationfollowing the base station B(n), that has now become the new currentbase station, i.e. base station B(n+l), is activated to receive in a newsafe transfer cycle beginning at step ET2.

It should be observed that in the infrastructure 1, following step ET2,only the UP data of the call COM received by the infrastructure 1 viathe current base station B(n-1) is actually retained or used for thepurpose of the call established between the mobile station M and aremote station, with the UP data DM of the call COM as received by theinfrastructure 1 via the following base station B(n) being used, incombination with the UP data of the call COM as received by theinfrastructure 1 via the current base station B(n-1), solely for thepurpose of determining a propagation time difference TA.

In the above description, and in particular with reference to step ET2,only one following base station B(n) is activated. Nevertheless, whenthe mobile station passes through a node rail station, a plurality offollowing base stations are activated in accordance with the invention.Thus, with reference to FIG. 1, for the example of a mobile stationtravelling from D to A, on passing through cell C100, it is appropriateto activate two following base stations associated with cells C20 andC10 respectively. Power measurement is then performed in each of theactivated following base station so that on expiry of a predeterminedduration, it is possible to release the following channel being usedwith that one of the following base stations which is not receiving theestablished call at sufficient power, i.e. in this example, the basestation associated with coverage area C10.

Each length of the transport network between the rail station(s) of nodeC and/or the terminus rail station(s) A, B, and D defines two paths inopposite directions which are carried by two respective tracks, such asAC and CA for the length interconnecting A and C. For a path from afirst mobile station carried by a track in a given direction, the methodof the invention requires a following channel to be available in thefollowing base station which is identical to the current channelcarrying the call with the mobile station. This means that it isnecessary to define two respective sets of channels for the twodirections, since otherwise it could happen that a mobile station in onetrain cannot benefit from a following channel in a following basestation that is identical to the current channel in the current basestation, assuming that the following channel is already in use by amobile station in a train travelling in the opposite direction. Theinvention remedies the above by proposing that in all base stations of alength of a transport network, a respective set of channels is allocatedto each of the two train travel directions. The infrastructure 1 is alsodesigned so that the channel to which the unsafe transfer is performed,i.e. the first transfer, starting from a source base station where acall is set up towards a target base station, is selected from arespective one of the two sets of channels as a function of the traveldirection of the mobile station. This travel direction is known to theinfrastructure firstly as a function of the source base station throughwhich the call is established, and secondly as a function of the targetbase station towards which channel handover is requested.

We claim:
 1. A method of making a safe handover of a call set up betweena mobile station using a current channel associated with a current basestation to a following channel associated with a following base station,along a transport network path associated in deterministic manner withsuccessive base stations in a mobile station radio network, the methodcomprising:activating a base station along the path as the followingbase station; receiving, at the following base station in the followingchannel, UP data of the call as transmitted in the current channel bythe mobile station, the current channel and the following channel beingthe same; measuring a propagation time difference between receipt of theUP data by the current base station in the current channel and by thefollowing base station in the following channel; simultaneouslytransmitting DOWN data for the call, during a given duration, from thecurrent base station over the current channel and from the followingbase station over the following channel, in response to a comparisonbetween the propagation time difference and a first predeterminedthreshold; and after the given duration, releasing the call in thecurrent channel, and validating the call in the following channel as anew current channel.
 2. The method according to claim 1, wherein thefollowing base station is determined as a function of:the current basestation, source and target base stations, between which an unsafehandover is performed, starting from the source base station throughwhich the call is set up, and a base station sequence stored in theinfrastructure of the radio network.
 3. The method according to claim 2,wherein two opposite direction paths are defined for a length of thetransport network, and wherein the unsafe handover of the call to thetarget base station is performed on a channel selected from one of twosets of channels, as a function of the travel direction of the mobilestation.
 4. The method according to claim 3, wherein the successive basestations associated with the path of the mobile station comprise all ofthe base stations between the target base station and a last basestation associated with a node or an end of the transport network. 5.The method according to claim 1, wherein the given duration ispredetermined and fixed.
 6. The method according to claim 1, wherein thegiven duration expires in response to a comparison between thepropagation time difference and a second predetermined threshold.